Clutch pedal drops to the floor with no real effort. How do I check the master cylinder?

Check for broken clutch cable if equipped or clutch master cylinder low/empty of fluid indicating a worn clutch or hydraulic leak

Your Clutch slave cylinder may be bad.here's a link to it. SAVE 20 GET 10 GIFT CARD ON ONLINE SHIP TO HOME ORDERS OF 100 OR MORE Here's a link to a new one. http://www.autozone.com/drivetrain/clutch-slave-cylinder/duralast-clutch-slave-cylinder/nissan-datsun/maxima/2005/6-cylinders-b-3-5l-sfi-dohc/557935_0_0

no fluid in the clutch reservoir due to a failed clutch slave cylinder
time to replace both the clutch slave and master cylinder

It's usually the clutch master or slave cylinder that is bad when the pedal drops to the floor suddenly. Check both to see which one is leaking fluid and that's your bad one.

Broken linkage, or the clutch itself has failed.

It is most likely a hydraulic clutch, and it is most likely leaking at the clutch slave cylinder. Check the clutch master cylinder for fluid, it should be a small master cylinder looking thing with a fill cap next to the brake master cylinder and booster unit. If it is empty fill it with the same dot 3 brake fluid that you use for your brake master cylinder, and make sure that you are pouring the fluid into the clutch master cylinder reservoir and not the rubber seal that should be under the fill cap. After filling the clutch master cylinder up with fluid, then try and pump up the clutch pedal. It might take a while to pump up hydraulic pressure, and you might also have to work the pedal a few times by hand at first.

Also, you might have to bleed the clutch at the slave cylinder if it will not build up enough pressure to release the clutch.

Need to have it check could be clutch master cylinder or slave cylinder or hose from clutch master cylinder to slave cylinder.check brake fluid at clutch master cylinder sit next to brake master cylinder if empty could have a leak. fill and pump peddle and look for leak.

try pulling pedal up, does it drop str8 to the floor,if yes problem in hydralic clutch like clutch master cylinder or slave cylinder,if pdel stays up appears to be ok then the clutch may need replacing

It's a hydraulic system on the Audi. Using a master cylinder (nearer pedal and firewall) and a slave cylinder (at clutch release bearing itself, or on side of tranny. Owner's manual should give location for a reservoir to check. Similar but smaller than brake master cylinder reservoir if that helps. The system uses brake fluid, and you either have a leak, a lack, or a loss of this fluid, and/or pressure to the clutch. Sounds like you need qualified service, these systems need REAL diagnosis before you start buying parts. ALso can be hard to bleed completely free of air after parts are installed.